Cold-rolled steel sheet with excellent shape fixability and method of manufacturing the same

ABSTRACT

A steel material having a chemical composition contains 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, whereby a resulting cold-rolled steel sheet has a microstructure dominated by ferrite with an average grain size of 10 μm to 30 μm, a proportional limit of 100 MPa or less, and excellent shape fixability.

TECHNICAL FIELD

This disclosure relates to a cold-rolled steel sheet suitable for members of parts requiring strict dimensional accuracy in the electrical, automotive, building material, and other fields and which has excellent shape fixability and also relates to a method of manufacturing the same. The disclosure particularly relates to the enhancement of shape fixability.

BACKGROUND

In recent years, to protect the global environment, reduction of automotive fuel consumption has been required from the viewpoint of reducing CO₂ emissions. For such a request to reduce fuel consumption, reduction in weight of automotive bodies has been attempted. Furthermore, demands to reduce the gauge of steel and the amount of steel have been growing in association with a requirement for cost reduction. However, reduction in gauge of steel materials (steel sheets) reduces the rigidity of parts to cause problems such as deflections, dents and warpage of the parts. Furthermore, in the field of consumer electrical appliances such as AV devices and OA machines, requirements for dimensional accuracy of parts have become strict and therefore demands for steel sheets with excellent shape fixability have been increasingly growing.

For such requirements, for example, WO 00/06791 discloses a ferritic steel sheet with excellent shape fixability. In a technique described in WO '791, steel having a composition containing 0.0001% to 0.05% C, 0.01% to 1.0% Si, 0.01% to 2.0% Mn, 0.15% or less P, 0.03% or less S, 0.01% or less Al, 0.01% or less N, and 0.007% or less O on a mass basis is hot-rolled such that the sum of rolling reductions at a temperature of not lower than the Ar₃ transformation temperature to 950° C. is 25% or more and the coefficient of friction during hot rolling at 950° C. or lower is 0.2 or less, hot rolling is completed at a temperature not lower than the Ar₃ transformation temperature, and coiling is performed at a temperature not higher than a predetermined critical temperature after cooling, whereby a steel sheet in which the ratio of the {100} plane to {111} plane parallel to a sheet surface is 1.0 or more is obtained. In the steel sheet, a slip system can be controlled during bending and springback can be suppressed during bending-dominated forming.

Japanese Unexamined Patent Application Publication No. 2002-66637 discloses a method of press-forming a formed product with excellent dimensional accuracy. In a technique described in JP '637, forming is performed using a steel sheet in which the ratio of the {100} plane to {111} plane parallel to a sheet surface is 1.0 or more such that a tensile stress equal to 40% to 100% of the tensile strength of material is applied to a vertical wall portion of a hat-shaped member. According to that technique, a member having significantly increased hat bendability, small springback, and excellent shape fixability can be provided.

However, the technique described in WO '791 has problems such as: the degree of improvement in shape fixability is small in performing press forming other than bending, and springback may be large due to the influence of grain boundary sliding or the like even when performing bending. Furthermore, the technique described in JP '637 has a problem that the effect of improving the dimensional accuracy of a formed product is not obtained in performing press forming other than hat forming and a problem that the blank holding pressure needs to be large to apply stress to a vertical wall in performing hat forming and therefore the power of a press needs to be increased, leading to an increase in cost.

It could therefore be helpful to provide a cold-rolled steel sheet having excellent shape fixability and causing no significant strain in a flat portion of a formed member and a method of manufacturing the same.

SUMMARY

We discovered that the strain of a flat portion of a formed member is significantly affected by the proportional limit of a steel sheet used. We also found that the strain of a flat portion of a formed member is significantly increased particularly when the proportional limit is more than 100 MPa. We further found that an ultra-low carbon based chemical composition essentially containing Ti and B needs to be adjusted such that the ratio, B/C, of the content of B to the content of C satisfies 0.5 or more such that the proportional limit is 100 MPa or less.

We thus provide:

-   -   (1) A cold-rolled steel sheet with excellent shape fixability,         has a chemical composition containing 0.0010% to 0.0030% C,         0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or         less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060%         Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C         satisfies 0.5 or more, the remainder being Fe and incidental         impurities; a microstructure dominated by ferrite with an         average grain size of 10 μm to 30 μm; and a proportional limit         of 100 MPa or less.     -   (2) The cold-rolled steel sheet specified in (1) further         contains 0.009% or less Nb on a mass basis in addition to the         chemical composition.     -   (3) The cold-rolled steel sheet specified in (1) further         contains 0.06% or less Cr on a mass basis in addition to the         chemical composition.     -   (4) The cold-rolled steel sheet specified in (1) further         contains 0.009% or less Nb and 0.06% or less Cr on a mass basis         in addition to the chemical composition.     -   (5) In the cold-rolled steel sheet specified in (2), the content         of Nb is 0.001% to 0.009% on a mass basis.     -   (6) In the cold-rolled steel sheet specified in (3), the content         of Cr is 0.001% to 0.06% on a mass basis.     -   (7) In the cold-rolled steel sheet specified in (1), the B/C is         greater than or equal to 0.5 and less than or equal to 5.     -   (8) In the cold-rolled steel sheet specified in (7), the B/C is         greater than or equal to 1.0 and less than or equal to 3.3.     -   (9) In the cold-rolled steel sheet specified in (8), the B/C is         greater than or equal to 1.5 and less than or equal to 3.3.     -   (10) In the cold-rolled steel sheet specified in (1), the         proportional limit is greater than or equal to 40 MPa and less         than or equal to 100 MPa.     -   (11) In the cold-rolled steel sheet specified in (1), the         microstructure dominated by ferrite contains 95% or more ferrite         in terms of area fraction.     -   (12) A method of manufacturing a cold-rolled steel sheet with         excellent shape fixability includes subjecting a steel material         to a hot-rolling step, a pickling step, a cold-rolling step, and         an annealing step in that order. The steel material has a         chemical composition containing 0.0010% to 0.0030% C, 0.05% or         less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S,         0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and         0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5         or more, the remainder being Fe and incidental impurities. The         hot rolling step is a step in which the steel material is         heated, is roughly rolled, is finish-rolled at a finishing         delivery temperature of 870° C. to 950° C., and is coiled at a         coiling temperature of 450° C. to 630° C. The cold-rolling step         is a step in which cold rolling is performed at a rolling         reduction of 90% or less. The annealing step is a step in which         heating is performed up to a holding temperature in the range of         700° C. to 850° C. at an average heating rate of 1° C./s to 30°         C./s in a temperature region not lower than 600° C., retention         is performed at the holding temperature for 30 s to 200 s, and         cooling is then performed at a cooling rate of 3° C./s or more         in a temperature region down to 600° C.     -   (13) In the method of manufacturing the cold-rolled steel sheet         specified in (12), it further contains 0.009% or less Nb on a         mass basis in addition to the chemical composition.     -   (14) In the method of manufacturing the cold-rolled steel sheet         specified in (12), it further contains 0.06% or less Cr on a         mass basis in addition to the chemical composition.     -   (15) In the method of manufacturing the cold-rolled steel sheet         specified in (12), it further contains 0.009% or less Nb and         0.06% or less Cr on a mass basis in addition to the chemical         composition.     -   (16) In the method of manufacturing the cold-rolled steel sheet         specified in (13), the content of Nb is 0.001% to 0.009% on a         mass basis.     -   (17) In the method of manufacturing the cold-rolled steel sheet         specified in (14), the content of Cr is 0.001% to 0.06% on a         mass basis.     -   (18) In the method of manufacturing the cold-rolled steel sheet         specified in (12), the B/C is greater than or equal to 0.5 and         less than or equal to 5.     -   (19) In the method of manufacturing the cold-rolled steel sheet         specified in (18), the B/C is greater than or equal to 1.0 and         less than or equal to 3.3.     -   (20) In the method of manufacturing the cold-rolled steel sheet         specified in (19), the B/C is greater than or equal to 1.5 and         less than or equal to 3.3.

A cold-rolled steel sheet having a significantly reduced proportional limit and excellent shape fixability after forming can be readily manufactured at low cost. This is industrially particularly advantageous. Furthermore, there is an effect that the reduction in gauge of a member can be accelerated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a test specimen for punch stretch forming and a flange-suppressing region (hatched portion) during a forming test.

FIG. 2 is a schematic view showing a method of measuring the maximum strain height after a punch stretch forming test.

FIG. 3 is a graph showing the relationship between the proportional limit and the maximum strain height.

FIG. 4 is a graph showing the relationship between B/C and the proportional limit.

DETAILED DESCRIPTION

First, reasons for limiting the composition (chemical composition) of a cold-rolled steel sheet are described. Incidentally, mass percent is hereinafter simply represented by % unless otherwise specified.

C: 0.0010% to 0.0030%

C is an element which forms a solid solution to promote formation of coarse B precipitates and which contributes to a reduction in proportional limit. Such an effect is remarkable when the content thereof is 0.0010% or more. However, when the content thereof is high, more than 0.0030%, the reduction of ductility is caused because the amount of solute C and/or carbides is large and the strength is excessively high. Therefore, C is limited to 0.0010% to 0.0030%. It is preferably 0.0020% or less.

Si: 0.05% or Less

When a large amount of Si is contained, workability is deteriorated by hardening, and Si oxides are produced during annealing and thereby wettability is impaired. Furthermore, since high Si content increases the austenite (γ)-to-ferrite (α) transformation temperature, it is difficult to complete rolling in a γ-region during hot rolling. Therefore, Si is 0.05% or less.

Mn: 0.1% to 0.5%

Mn combines with S, where S significantly reduces hot ductility and is harmful, in steel to form MnS, contributes to rendering S harmless, and has the effect of hardening steel. The content thereof needs to be 0.1% or more to achieve such effects. However, when the content thereof is high, more than 0.5%, ductility is reduced by hardening and recrystallization of ferrite is suppressed during annealing. Therefore, Mn is 0.1% to 0.5%. It is preferably 0.3% or less and more preferably 0.2% or less.

P: 0.05% or Less

P segregates at grain boundaries and has the function of reducing ductility. Therefore, P is preferably minimized and up to 0.05% is acceptable. Hence, P is 0.05% or less. It is preferably 0.03% or less and more preferably 0.02% or less.

S: 0.02% or Less

S is an impurity element and is preferably minimized. S significantly reduces hot ductility, causes hot cracking, significantly deteriorates surface properties, and has adverse influences. Furthermore, S hardly contributes to strength and forms coarse MnS to reduce ductility. This becomes significant when S is more than 0.02%. Therefore, S is 0.02% or less. It is preferably 0.01% or less.

Al: 0.10% or Less

Al is an element acting as a deoxidizer. 0.02% or more is preferably contained to achieve such an effect. On the other hand, Al has the function of increasing the γ-to-α transformation temperature of steel. Therefore, when the content is high, more than 0.10%, it is difficult to complete rolling in a γ-region during hot rolling. Therefore, Al is 0.10% or less.

N: 0.0050% or Less

N is an element which combines with a nitride-forming element to form a nitride and has the function of hardening steel by precipitation hardening. When the content is high, more than 0.0050%, not only a reduction in ductility but also slab cracking during hot rolling are caused and many surface flaws may possibly be caused. Therefore, N is 0.0050% or less. It is preferably 0.0030% or less and more preferably 0.0020% or less.

Ti: 0.021% to 0.060%

Ti is an element which fixes N in the form of a nitride and has the function of suppressing hardening and aging deterioration due to solute N. 0.021% or more needs to be contained to achieve such effects. However, when the content is high, more than 0.060%, the precipitation of carbides is promoted and the amount of solute C is reduced. Hence, production of coarse B precipitates containing C and Fe is suppressed. Therefore, a desired reduction in proportional limit cannot be achieved. Thus, Ti is 0.021% to 0.060%. It is preferably 0.050% or less.

B: 0.0005% to 0.0050%

B is an important element and forms coarse B precipitates to contribute to a reduction in proportional limit. 0.0005% or more needs to be contained to achieve such an effect. However, when the content is high, more than 0.0050%, slab cracking is caused. Therefore, B is 0.0005% to 0.0050%. It is preferably 0.0010% or more, more preferably 0.0020% or more, and further more preferably 0.0030% or more.

B/C: 0.5 or More

C and B are contained in the above ranges and the contents of C and B are adjusted such that the ratio, B/C, of the content of B to the content of C satisfies 0.5 or more. When B/C is less than 0.5, it is difficult to form coarse B precipitates. Therefore, B/C is limited to 0.5 or more. Incidentally, it is preferably 1.0 or more, more preferably 1.5 or more, and further more preferably 2.0 or more.

The above components are fundamental components. 0.009% or less Nb and/or 0.06% or less Cr may be contained as a selective element in addition to the fundamental components as required.

Nb: 0.009% or Less

Nb, as well as Ti, is an element which combines with N to form a nitride, which fixes N, which suppresses hardening and aging deterioration due to solute N, and contributes to enhancement of shape fixability and may be contained as required. 0.001% or more is preferably contained to achieve such effects. However, the content is high, more than 0.009%, grains become fine. Therefore, when Nb is contained, Nb is preferably 0.009% or less.

Cr: 0.06% or Less

Cr is an element which destabilizes C in a solid solution to promote production of coarse B precipitates containing C and may be contained as required. 0.001% or more is preferably contained to achieve such an effect. However, when the content of Cr is high, more than 0.06%, the production of the coarse B precipitates containing C is inhibited instead. Therefore, when Cr is contained, Cr is preferably 0.06% or less. The remainder other than the above components are Fe and incidental impurities.

Next, reasons for limiting the microstructure of the cold-rolled steel sheet are described.

The cold-rolled steel sheet has a microstructure dominated by ferrite with an average grain size of 10 μm to 30 μm. The microstructure dominated by ferrite allows the steel sheet to be soft and therefore allows workability thereof to be enhanced. The term “microstructure dominated by ferrite” as used herein refers to a microstructure in which ferrite (polygonal ferrite) accounts for 95% or more, and more preferably 100%, in terms of area fraction. A secondary phase other than ferrite is preferably cementite or bainite. If the average grain size of ferrite is 10 μm or more, the concentration of strain at grain boundaries can be suppressed, strain can be concentrated around precipitates, and the proportional limit can be reduced. However, when the average grain size of ferrite is large, more than 30 μm, surface markings such as orange peeling become obvious during press working. Therefore, the average grain size of ferrite is 10 μm to 30 μm. It is preferably 15 μm to 25 μm.

Next, a preferred method of manufacturing the cold-rolled steel sheet is described.

A steel material (slab) with the above composition is used as a starting material.

A method of manufacturing the steel material is not particularly limited. Molten steel with the above composition is preferably produced in a regular converter, an electric furnace, or the like and is then solidified into a slab (steel material) by a continuous casting process or an ingot casting-blooming process. If the slab is manufactured by continuous casting, the slab is preferably directly hot-rolled without cooling the slab to room temperature when having heat sufficient for hot rolling. Alternatively, the slab is preferably hot-rolled after the slab is temporally charged into a furnace and is heat-retained or the slab is cooled to room temperature and is then reheated to a temperature of 1,100° C. to 1,250° C. by charging the slab into a furnace.

The heated steel material is subjected to a hot rolling step.

In the hot rolling step, hot rolling including rough rolling and finish rolling is performed and coiling is then performed.

In rough rolling, conditions are not particularly limited as far as a sheet bar having a desired size and shape is obtained. Next, the sheet bar is finish-rolled, whereby a hot-rolled sheet is obtained.

Finish rolling is performed at a finishing delivery temperature of 870° C. to 950° C.

When the finishing delivery temperature is low, lower than 870° C., the microstructure is transformed from austenite into ferrite in the course of rolling and therefore it is difficult to control the load of a rolling machine. Hence, the risk of causing fracture or the like during processing increases. Incidentally, if rolling is performed from the finishing entry side in a ferrite region, the fracture or the like during processing can be avoided. However, there is a problem in that the microstructure of the hot-rolled sheet is transformed into unrecrystallized ferrite because of the decrease of the rolling temperature and therefore the load for cold rolling is increased. On the other hand, when the finishing delivery temperature is high, higher than 950° C., the hot-rolled sheet has a large ferrite grain size. Therefore, a cold-rolled annealed sheet has an excessively large ferrite grain size. Thus, the finishing delivery temperature is 870° C. to 950° C. After finish rolling is completed, the hot-rolled sheet is coiled. Cooling until coiling after finish rolling is not particularly limited and it is sufficient that the rate of cooling is higher than that of air cooling. There is no particular problem even if quenching is performed at 100° C./s or more as required.

The coiling temperature after the completion of finish rolling is 450° C. to 630° C.

When the coiling temperature is lower than 450° C., acicular ferrite is produced and a steel sheet is hardened. Hence, the load for subsequent cold rolling is increased and, also, leads to the difficulty in operating hot rolling. However, when the coiling temperature is high, higher than 630° C., the precipitation of carbides is promoted, the amount of solute C is reduced and, therefore, a desired amount of solute C cannot be ensured during hot rolling process. Thus, the coiling temperature is 450° C. to 630° C.

The coiled hot-rolled sheet is subjected to an ordinary pickling step and then subjected to a cold-rolling step, whereby a cold-rolled sheet is obtained.

In the cold-rolling step, the cold-rolled sheet is obtained by performing cold rolling at a cold-rolling reduction of 90% or less.

When the cold-rolling reduction is large, more than 90%, recrystallized ferrite grains after annealing become fine. At the same time, the load for cold rolling is increased, leading to difficulty in operating cold rolling. Thus, the cold-rolling reduction is limited to 90% or less. It is preferably 80% or less. The lower limit of the cold-rolling reduction is not particularly limited. However, when the cold rolling reduction is low, the thickness of the hot-rolled sheet needs to be reduced with respect to the predetermined thickness of products and, therefore, productivity of hot rolling and pickling is reduced. Hence, the cold-rolling reduction is preferably 50% or more.

The cold-rolled sheet is subjected to an annealing step, whereby a cold-rolled annealed sheet is obtained.

The annealing step is a step in which heating is performed up to a holding temperature of 700° C. to 850° C. at an average heating rate of 1° C./s to 30° C./s in a temperature region not lower than 600° C., retention is performed at the holding temperature for 30 s to 200 s, and cooling is then performed at a cooling rate of 3° C./s or more down to 600° C. or lower. In the annealing step, cold-rolled worked ferrite is recrystallized to have a desired average grain size and coarse B precipitates containing C and Fe are distributed at grain boundaries and in grains. Heating rate: 1° C./s to 30° C./s

When the average heating rate in a temperature region ranging from 600° C. to the holding temperature is less than 1° C./s, ferrite grains grow significantly and therefore ferrite with a desired average grain size cannot be obtained. However, when the heating rate is high, more than 30° C./s, TiC is precipitated during heating instead of the production of B precipitates and therefore it is difficult to form desired coarse B precipitates. Thus, the average heating rate in a temperature region not lower than 600° C. is limited to 1° C./s to 30° C./s. It is preferably 5° C./s or more and more preferably 10° C./s or more.

Holding Temperature: 700° C. to 850° C.

In the annealing step, the holding temperature is 700° C. or higher because the recrystallization of cold-worked ferrite needs to be completed. However, when the holding temperature is high, higher than 850° C., ferrite grains become coarse and therefore ferrite with a desired average grain size cannot be obtained. Thus, the holding temperature is 700° C. to 850° C.

Holding Time: 30 s to 200 s

The holding time is 30 s or more to complete the recrystallization of cold-worked ferrite. When the holding time is short, the recrystallization thereof is not completed or ferrite grains remain fine. However, when the holding time is long, more than 200 s, ferrite grains grow excessively. Thus, the holding time is of 30 s to 200 s.

Cooling Rate: 3° C./s or More

Growth of ferrite grains is promoted when the cooling rate after holding is low. Thus, the average cooling rate in a temperature region ranging from the holding temperature to 600° C. is 3° C./s or more. The upper limit of the cooling rate need not be particularly limited and is determined depending on the capacity of a cooling facility. In ordinary cooling facilities, the upper limit of the cooling rate is about 30° C./s.

Coarsening of the microstructure due to growth of ferrite grains can be suppressed by cooling to 600° C., whereby a microstructure dominated by ferrite with a desired average grain size can be obtained. Conditions for cooling to 600° C. or less need not be particularly limited and arbitrary cooling is not particularly problematic.

After cooling is stopped, galvanizing may be performed at about 480° C. as required. After galvanizing, galvannealing may be performed by reheating to 500° C. or higher. Thermal history including retention during cooling may be performed. Furthermore, temper rolling may be performed at about 0.5% to 2% as required. When not performing plating, electrogalvanizing may be performed for the purpose of enhancing corrosion resistance. Furthermore, a coating may be provided on the cold-rolled steel sheet or a plated steel sheet using chemical conversion or the like.

Our steel sheets and methods are further described below in detail on the basis of examples.

Examples

First, experiment results underlying our steel sheets and methods are described.

Steel materials (slabs) having a composition containing 0.0010% to 0.035% C, 0.01% to 0.03% Si, 0.10% to 0.45% Mn, 0.03% to 0.08% Al, 0.022% to 0.060% Ti, 0.0003% to 0.0048% B, and 0.0015% to 0.0040% N on a mass basis were subjected to hot rolling and cold rolling and further subjected to annealing under various heating, holding, and cooling conditions, whereby cold-rolled annealed sheets were obtained.

A JIS #5 test specimen was taken from each obtained cold-rolled annealed sheet such that a tensile direction coincided with a rolling direction, followed by determining the proportional limit thereof. A 5 mm strain gauge was attached to a parallel portion of the tensile test specimen and tensile testing was performed at a cross head speed of 1 mm/min. The stress at which the slope of the stress-strain curve thereof began to decrease was defined as the proportional limit thereof.

A test specimen (a size of 120 mm×120 mm) was taken from each obtained cold-rolled annealed sheet and then punch stretch formed. Punch stretch forming was performed by press forming such that a central portion of the test specimen was stretched by 8 mm using a spherical punch with a diameter of 20 mm. In punch stretch forming, a region (hatched portion) with a diameter of 28 mm to 54 mm was pressed with a load of 100 kN and formed as shown in FIG. 1. Next, as shown in FIG. 2, the formed test specimen was placed on a platen and a flange portion thereof was measured for maximum strain height. Observation of the obtained cold-rolled annealed sheets showed that all the cold-rolled annealed sheets had a microstructure dominated by ferrite.

The obtained results are shown in FIGS. 3 and 4. FIG. 3 shows the relationship between the proportional limit and maximum strain height of each flange portion. FIG. 4 shows the relationship between B/C and the proportional limit.

As is clear from FIG. 3, as the proportional limit exceeds 100 MPa, the maximum strain height of the flange portion increases sharply. As is clear from FIG. 4, to adjust the proportional limit to 100 MPa or less, B/C needs to be 0.5 or more.

From this, we found that the shape fixability of a pressed part is increased and particularly the strain of a flat portion of a formed member is significantly reduced by using a steel sheet having a composition which essentially contains Ti and B and in which B/C is 0.5 or more, a microstructure dominated by ferrite, and a proportional limit of 100 MPa or less as material. We also found that it is effective in enhancing shape fixability that hot rolling conditions are controlled such that C forms a solid solution, cold rolling is performed, and coarse B precipitates containing C and Fe are formed at grain boundaries and also in grains during annealing. We further found that, in such a microstructure, distributed coarse B precipitates adequately anchor dislocations during press forming to concentrate strain around the precipitates and suppress intertwining of dislocations by preventing the dislocations from gathering at grain boundaries, whereby springback is significantly reduced, the proportional limit is reduced, and shape fixability is remarkably enhanced.

Then, in particular Examples, steel materials (slabs) having a chemical composition shown in Table 1 were used as starting materials. After the slabs were heated to 1,200° C., the slabs were subjected to a hot-rolling step, a pickling step, a cold-rolling step, and an annealing step in that order, whereby cold-rolled annealed sheets were obtained. In the hot-rolling step, each steel material was roughly rolled into a sheet bar and the sheet bar was finish-rolled at a finishing delivery temperature equal to a temperature (FT) shown in Table 2 and was then coiled at a coiling temperature (CT) shown in Table 2, whereby a hot-rolled sheet with a thickness shown in Table 2. Next, after the hot-rolled sheet was subjected to the pickling step, the hot-rolled sheet was subjected to cold rolling at a cold-rolling reduction shown in Table 2, whereby a cold-rolled sheet with a thickness shown in Table 2 was obtained.

Next, the cold-rolled sheet was subjected to the annealing step, whereby a cold-rolled annealed sheet was obtained. In the annealing step, annealing was performed at a heating rate, a holding temperature, a holding time, and a cooling rate as shown in Table 2. Cooling from 600° C. or lower to room temperature was performed at a similar cooling rate. After the annealing step was performed, temper rolling was performed at a rolling reduction of 1.0%.

The obtained cold-rolled annealed sheets (cold-rolled steel sheets) were subjected to microstructure observation, a tensile test, and a punch stretch forming test. Testing methods were as described below.

(1) Microstructure Observation

A test specimen for microstructure observation was taken from each obtained cold-rolled annealed sheet; a cross section (L-cross section) in a rolling direction was polished and etched; the microstructure thereof was observed and photographed using an optical microscope (a magnification of 100 times) and a scanning electron microscope (a magnification of 1,000 times); and the average grain size of ferrite, the fraction of ferrite, and the type and fraction of a secondary phase were determined by image analysis. For ferrite, the average intercept length of ferrite grains in a 300 μm×300 μm region was determined in the rolling and thickness directions and the value of 2/(1/A+1/B) was defined as the average grain size, where A is the average intercept length of the ferrite grains in the rolling direction and B is the average intercept length of the ferrite grains in the thickness direction. The fraction of ferrite was measured in a 300 μm×300 μm region.

(2) Tensile Test

A JIS #5 test specimen was taken from each obtained cold-rolled annealed sheet such that a tensile direction coincided with the rolling direction, followed by determining the proportional limit thereof. A strain gauge was attached to a parallel portion of the tensile test specimen and tensile testing was performed at a cross head speed of 1 mm/min, whereby tensile properties (proportional limit, tensile strength, and elongation) were determined. Incidentally, the proportional limit was defined as the stress at which the slope of the stress-strain curve thereof began to decrease.

(3) Punch Stretch Forming Test

A test specimen (a size of 120 mm×120 mm) was taken from each obtained cold-rolled annealed sheet and was then punch stretch formed. Punch stretch forming was performed by press forming such that a central portion of the test specimen was stretched by 8 mm using a spherical punch with a diameter of 20 mm. In punch stretch forming, a region (hatched portion) with a diameter of 28 mm to 54 mm was depressed with a load of 100 kN and formed as shown in FIG. 1. After forming, as shown in FIG. 2, the test specimen was placed on a platen and a flange portion thereof was measured for maximum strain height. Obtained results are shown in Table 3.

TABLE 1 Chemical components (weight percent) Steel Material ID C Si Mn P S Al N Ti B Nb Cr B/C Remarks A 0.0015 0.01 0.15 0.01 0.01 0.03 0.0020 0.040 0.0029 — — 1.9 Adequate Example B 0.0013 0.03 0.35 0.04 0.01 0.05 0.0040 0.022 0.0018 0.005 1.4 Adequate Example C 0.0016 0.02 0.45 0.02 0.02 0.08 0.0030 0.058 0.0009 0.008 0.01 0.6 Adequate Example D 0.0028 0.05 0.25 0.01 0.01 0.04 0.0020 0.035 0.0048 — 0.05 1.7 Adequate Example E 0.0012 0.01 0.15 0.01 0.01 0.05 0.0015 0.031 0.0025 — — 2.1 Adequate Example F 0.0013 0.01 0.15 0.01 0.01 0.04 0.0025 0.055 0.0035 — 0.01 2.7 Adequate Example G 0.0012 0.01 0.10 0.01 0.01 0.05 0.0015 0.060 0.0040 — — 3.3 Adequate Example H 0.0025 0.01 0.10 0.01 0.01 0.04 0.0020 0.035 0.0023 — — 0.9 Adequate Eample I 0.0015 0.01 0.15 0.01 0.01 0.05 0.0020 0.045 0.0008 — — 0.5 Adequate Example J 0.0035 0.02 0.25 0.02 0.01 0.05 0.0025 0.032 0.0015 — — 0.4 Comparative Example K 0.0010 0.01 0.20 0.02 0.01 0.06 0.0021 0.025 0.0003 — — 0.3 Comparative Example L 0.0020 0.01 0.18 0.01 0.02 0.05 0.0023 0.035 0.0010 0.003 — 0.5 Adequate Example M 0.0011 0.02 0.15 0.02 0.01 0.04 0.0030 0.030 0.0020 — — 1.8 Adequate Example N 0.0025 0.02 0.20 0.01 0.01 0.04 0.0030 0.040 0.0020 — — 0.8 Adequate Example O 0.0015 0.01 0.15 0.01 0.01 0.04 0.0030 0.005 0.0030 — — 2.0 Comparative Example

TABLE 2 Hot-rolling step Cold-rolling step Annealing step Finishing Cold- Heating Steel Steel Heating delivery Coiling Thick- rolling Thick- rate Holding Holding Cooling sheet Material temperature temperature temperature ness reduction ness (° C./ temperature time rate ID No. (° C.) (° C.) (° C.) (mm) (%) (mm) s)* (° C.) (s) (° C./s)** Remarks 1 A 1200 890 560 2.5 76 0.6 11 770 130 20 Example 2 B 1200 920 620 2.7 78 0.6 6 720 40 5 Example 3 C 1200 940 460 1.5 60 0.6 3 840 180 12 Example 4 D 1200 900 500 1.3 55 0.6 20 780 80 25 Example 5 E 1200 890 600 2.0 70 0.6 28 800 100 15 Example 6 F 1200 930 580 2.4 75 0.6 15 830 150 10 Example 7 G 1200 920 570 2.9 79 0.6 12 850 180 8 Example 8 H 1200 910 580 2.4 75 0.6 10 800 150 10 Example 9 I 1200 890 560 2.7 78 0.6 10 800 130 18 Example 10 J 1200 890 600 2.4 75 0.6 12 830 130 10 Comparative Example 11 K 1200 880 590 2.5 76 0.6 10 820 120 11 Comparative Example 12 L 1200 910 650 2.7 78 0.6 15 800 140 15 Comparative Example 13 M 1200 890 590 2.4 75 0.6 0.4 860 150 10 Comparative Example 14 N 1200 880 560 2.2 73 0.6 12 750 20 15 Comparative Example 15 O 1200 890 560 2.4 75 0.6 10 750 100 15 Comparative Example *Average in a temperature region not lower than 600° C. **Average from a holding temperature to 600° C.

TABLE 3 Steel Microstructure Tensile properties Shape fixability sheet Ferrite Proportional limit Tensile strength Elongation Maximum strain No. Type* Average grain size (μm) Fraction (area percent) (MPa) TS (MPa) El (%) height (mm) Remarks 1 F 16 100 80 330 50 0.4 Example 2 F 12 100 85 340 49 0.6 Example 3 F + C 11 98 100 350 48 0.7 Example 4 F 13 100 80 355 47 0.5 Example 5 F 16 100 70 320 51 0.3 Example 6 F 23 100 50 310 51 0.2 Example 7 F 28 100 40 300 52 0.2 Example 8 F 12 100 95 330 50 0.7 Example 9 F 13 100 100 320 51 0.8 Example 10 F 10 100 125 360 46 2.0 Comparative Example 11 F 12 100 130 320 51 2.2 Comparative Example 12 F 11 100 120 340 49 1.9 Comparative Example 13 F 35 100 100 290 53 0.8 Comparative Example 14 F + C 8 97 130 330 50 2.3 Comparative Example 15 F 15 100 140 340 48 2.4 Comparative Example *F represents ferrite, C represents cementite, and B represents bainite.

In all our Examples, cold-rolled steel sheets have excellent shape fixability with a low proportional limit of 100 MPa or less and flat portions of punch stretch formed members having a maximum strain height of 0.8 mm or less. However, in Comparative Examples which are outside our scope, the proportional limit is more than 100 MPa or the maximum strain height is large, more than 0.8 mm, and shape fixability is low. 

1-20. (canceled)
 21. A cold-rolled steel sheet with excellent shape fixability, having a chemical composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities; a microstructure dominated by ferrite with an average grain size of 10 μm to 30 μm and a proportional limit of 100 MPa or less.
 22. The cold-rolled steel sheet according to claim 21, further containing 0.009% or less Nb on a mass basis in addition to the chemical composition.
 23. The cold-rolled steel sheet according to claim 21, further containing 0.06% or less Cr on a mass basis in addition to the chemical composition.
 24. The cold-rolled steel sheet according to claim 21, further containing 0.009% or less Nb and 0.06% or less Cr on a mass basis in addition to the chemical composition.
 25. The cold-rolled steel sheet according to claim 22, wherein the content of Nb is 0.001% to 0.009% on a mass basis.
 26. The cold-rolled steel sheet according to claim 23, wherein the content of Cr is 0.001% to 0.06% on a mass basis.
 27. The cold-rolled steel sheet according to claim 21, wherein the B/C is greater than or equal to 0.5 and less than or equal to
 5. 28. The cold-rolled steel sheet according to claim 27, wherein the B/C is greater than or equal to 1.0 and less than or equal to 3.3.
 29. The cold-rolled steel sheet according to claim 28, wherein the B/C is greater than or equal to 1.5 and less than or equal to 3.3.
 30. The cold-rolled steel sheet according to claim 21, wherein the proportional limit is greater than or equal to 40 MPa and less than or equal to 100 MPa.
 31. The cold-rolled steel sheet according to claim 21, wherein the microstructure dominated by ferrite contains 95% or more ferrite in terms of area fraction.
 32. A method of manufacturing a cold-rolled steel sheet with excellent shape fixability, comprising subjecting a steel material to a hot-rolling step, a pickling step, a cold-rolling step, and an annealing step in that order, wherein the steel material has a composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities; the hot rolling step is a step in which the steel material is heated, roughly rolled, finish-rolled at a finishing delivery temperature of 870° C. to 950° C., and coiled at a coiling temperature of 450° C. to 630° C.; the cold-rolling step is a step in which cold rolling is performed at a rolling reduction of 90% or less; and the annealing step is a step in which heating is performed up to a holding temperature in the range of 700° C. to 850° C. at an average heating rate of 1° C./s to 30° C./s in a temperature region not lower than 600° C., retention is performed at the holding temperature for 30 s to 200 s, and cooling is performed at a cooling rate of 3° C./s or more in a temperature region down to 600° C.
 33. The method according to claim 32, wherein the chemical composition further contains 0.009% or less Nb on a mass basis.
 34. The method according to claim 32, wherein the chemical composition further contains 0.06% or less Cr on a mass basis.
 35. The method according to claim 32, wherein the chemical composition further contains 0.009% or less Nb and 0.06% or less Cr on a mass basis.
 36. The method according to claim 33, wherein the content of Nb is 0.001% to 0.009% on a mass basis.
 37. The method according to claim 34, wherein the content of Cr is 0.001% to 0.06% on a mass basis.
 38. The method according to claim 32, wherein the B/C is greater than or equal to 0.5 and less than or equal to
 5. 39. The method according to claim 38, wherein the B/C is greater than or equal to 1.0 and less than or equal to 3.3.
 40. The method according to claim 39, wherein the B/C is greater than or equal to 1.5 and less than or equal to 3.3. 